1. Field of the Invention
The present invention relates to a camera and, more particularly, to a temperature measuring apparatus for a camera, which compensates a shutter driving voltage or a lens extending amount in accordance with the temperature.
2. Description of the Related Art
Recently, plastic lenses or plastic barrels have been widely used in cameras. In a camera which performs focus adjustment by extending a lens, however, a focal point may be shifted due to a change in shape of a plastic lens caused by a temperature change because a plastic material has a large thermal expansion coefficient. As a method of preventing such a shift in focal point, Published Unexamined Japanese Patent Application No. 57-64204 proposes a technique of detecting the ambient temperature and compensating a change in focal length of a photographic lens caused by a temperature change in accordance with the detected temperature, thereby obtaining a proper focal point.
In addition, when a gear train formed by molding is used in a camera which controls shutter driving by an open loop, a change in friction coefficient of the gear train caused by temperature change has a direct effect on an aperture waveform. As a means for preventing this inconvenience, there is a conventionally known technique of maintaining an aperture waveform constant by compensating a shutter driving voltage or a driving duty ratio in accordance with the temperature. To properly perform compensation in accordance with a temperature change, the temperature of a plastic lens or a plastic barrel must be accurately measured. A thermistor can be used as a means for measuring the temperature. To reduce the manufacturing cost of a camera, however, a temperature measuring circuit is often incorporated in a controlling integrated circuit (IC) for controlling a shutter motor or a lens motor of a camera to perform compensation assuming that a temperature measured by the temperature measuring circuit is the temperature of a lens or a barrel.
When an actuator is driven, however, the temperature of the IC is increased by generated heat. Therefore, even when the temperature is measured by using the temperature measuring circuit incorporated in the IC, since a difference between the measured temperature in the IC and the temperature of a lens or a barrel is large, no accurate compensation can be performed.
To solve the above problem, Published Unexamined Japanese Patent Application No. 2-173711 proposes a method of measuring a temperature after a release button is depressed and before an actuator is driven for a photographing operation and performing the above compensation on the basis of the measurement value. In this method, a timer is activated after the operation of the actuator is finished, and temperature measurement is permitted only when the release button is depressed after a predetermined time period elapses from the end of operation of the actuator.
In the method proposed in Published Unexamined Japanese Patent Application No. 2-173711, however, if compensation according to the temperature must be performed before driving of the actuator, e.g., if an extending amount of a photographic lens, or a photometric value or a distance measurement value must be compensated in accordance with the temperature, or if a driving voltage for an optical system or a shutter must be compensated in accordance with the temperature, temperature measurement is performed after the release button is depressed, and then the actuator is driven. Therefore, a new problem arises in which a time lag from depression of the release button to start of photographing is increased by a time period required for temperature measurement.
In addition, in a camera which determines operation conditions such as exposure of the camera on the basis of obtained temperature data and displays the determination results, the temperature data indicates the temperature measured immediately before an immediately preceding operation of an actuator, whereas temperature data used in actual compensation indicates the temperature measured after a release button is depressed. Therefore, a displayed exposure value is sometimes different from an actual exposure value.